The present invention relates to a release coating composition that may be applied to a film that may then be used as a substrate useful for applications requiring release for a broad range of temperatures and high humidity conditions, which temperatures may range from about 20xc2x0 C. to about 210xc2x0 C. These applications include release substrate used in the manufacture of calendared cured sheet rubber and molding paste composites, such as sheet molding compound (SMC), thick molding compound (TMC), bulk molding compound (BMC) and fiberglass composites.
In the rubber industry, sheets of cured rubber compound are prepared by a calendaring process. Typically these sheets are from about 100 to about 400 feet in length. The uncured rubber sheet is laid onto a supporting interleaf film or sheet and then the two sheets are wound onto a mandrel. The interleaf is usually cellophane or silicon coated paper. The interleaf does not melt at the curing temperature and prevents the sheets from fusing with each other during the curing process. Sometimes talc or zinc stearate is applied to the interleaf to enhance release of rubber sheets from the interleaf after curing. Subsequently, the roll of rubber and interleaf can be over wound and held under tension using an over-wrap, which can be any film or cloth having good tensile properties that tends to shrink at oven curing temperatures. The cured sheet rubber may be used as components for aircraft engines and gaskets for rubber roofing membranes. Teflon(copyright) sheets, talc dust, and cloth are commonly used as interleaves in the rubber industry.
SMC is a composite material and usually comprises crosslinkable polymeric resin, most often unsaturated polyester resin; styrene monomer, plus catalyst; particulate filler, such as calcium carbonate; chopped glass fiber reinforcement; and various other additives in minor amounts, such as pigments and other modifiers.
The manufacture of SMC begins by laying the paste comprising all ingredients except the glass fibers, on a bottom carrier or release sheet, i.e., a film. The glass fibers are poured on top of the resin. More paste is poured over the glass fibers. A top carrier release sheet is laid down, and the edges of the top and bottom sheets are folded over to form a sandwich. The film and hence the composite is then kneaded to mix the glass fibers and the paste. The sandwich is then festooned (folded back and forth in a continuous fashion) into a bin and stored for up to about 14 days to cure or mature. Satisfactory results may be obtainable after as little as 2.5 days, but often more time is required. During this time the viscosity of the composite increases significantly (approximately ten-fold).
At the end of the curing period, the carrier release films, top and bottom are stripped away, the solidified SMC is cut and put into a heated press. In roughly one minute or less, out comes a semi-finished product, such as an auto part, for example, an automobile hood.
TMC is produced by a different machine and a process different from those used for producing SMC. Although TMC is prepared as a continuous length of material, it is cut into slabs for curing and storage because it is thicker than SMC. SMC is usually 1xe2x80x3 thick, but may range from xc2xcxe2x80x3 to 3xe2x80x3 in thickness. TMC may range from xc2xdxe2x80x3 to 4xe2x80x3 in thickness. TMC is stronger because some of its fiberglass fibers may be positioned vertically, and more filler may be added. A most significant difference between SMC and TMC is that in making TMC, the glass fibers are mixed with the paste prior to being deposited on the carrier or release film, and thus no kneading of the composite sandwich is necessary when TMC is made into slabs. This therefore places different requirements on the carrier or release film as tear strength may not be as critical for carrier release film used to make TMC.
BMC is also a composite material of resins, fillers and reinforcements. Typically, it comprises 30% resins, 50% fillers and additives and 20% reinforcement, such as glass fiber. It may also contain catalysts. The high filler loadings can provide improved stiffness and fire retardence. BMC is manufactured by preparing a putty-like molding compound comprising the above-noted components in a xe2x80x9cready to moldxe2x80x9d form. Molding pressures usually range from about 350 to 2000 psi at temperatures of between 250 and 350xc2x0 F. BMC can be made into precise shapes with various types of inserts, and therefore the moldings can be extremely complex. One limitation of BMC is the loss of strength caused by degradation of glass fiber reinforcements during energy-intensive mixing.
BMC is primarily used as a replacement for cast metals. The actual physical characteristics of BMC are determined primarily by the choice of resin and desired end use. Possible end uses include electrical grade; low shrink/general purpose; appliance/structural; low profile; automotive grade; and corrosion resistant. Major applications of BMC include air conditioner components; pump housings; circuit breakers; computer and business equipment components; garbage disposal housings; motor parts; power tools; gear cases; electrical insulators; and circuit covers.
In selecting a carrier release film there are some basic requirements or properties that are preferably met for the film to be suitable. While styrene barrier, moisture barrier, and mechanical strength are relevant, most important are release from the paste composite, be it SMC, BMC, or TMC, and the cost of the release film.
Nylon films represent a potential replacement for silicon-coated paper and cellophane as interleaves in the rubber calendaring industry, because of their high tensile strength. However, the tendency of currently manufactured nylon films to stick to rubber compounds both cured and uncured limits their use in a rubber release application. Apart from sticking to the sheets of rubber, the latter film sometimes causes wrinkles on the surface of the cured rubber. It is speculated that gases emanated during curing of rubber cause such wrinkles.
Cellulose ethers are water-soluble polymers derived from cellulose. A commercially available cellulose ether is available under the Methocel(copyright) brand from The Dow Chemical Company. These products are available in various viscosity grades, ranging from 3 to over 200,000 mPa""s. Generally, these viscosities refer to the viscosity of a 2% Methocel(copyright) solution in water at 25xc2x0 C. The methylcellulose products include hydroxypropyl substituted cellulose ethers. Such products are also available from other sources such as China Yixing Kaili Chemical Pharmaceutical Factory of Yixing city, Jiangsu, China; Carbomer Inc of Westborough, Mass.; and Penta Mnfg. Co. of Livingston, N.J. Methocel(copyright) products are used as mold-release agents, stabilizers, and thickeners in rubber latexes, where they contribute also to more uniform drying and less pinholing (see Dow METHOCEL(copyright) Cellulose Ethers Technical Handbook available from The Dow Chemical Company Website, July 2000).
Various attempts have been made to make and coat non-stick coatings to film or film structures used for high temperature applications. Some of the prior art patents pertaining to release coatings are summarized hereafter:
U.S. Pat. No. 5,139,835 to Kitamura et al discloses a synthetic resin laminated paper which makes it possible to recover paper (or laminated film) materials easily and rationally. The adhesion-release control agent layer interposed between the polyethylene film and paper layer can be polyvinyl alcohol, silicone based compound, or a reaction product of an organopolysiloxane compound having at least one double bond which has reacted with said hydrogen atom.
U.S. Pat. No. 3,503,773 to Bisschops et al discloses a process for forming films or foils using a high-gloss-surface or the xe2x80x9ccasting layerxe2x80x9d. The film-forming polymer solution is applied to the casting layer and at the end of the process the polymer film is stripped off the casting layer. The casting layer is a mixture of cellulose acetate and Werner chromium complex salt.
U.S. Pat. No. 4,956,233 to Chu et al discloses a slip-coated thermoplastic film having good antiblocking properties. The slip coating comprises of an aqueous wax emulsion or dispersion and a minor amount of talc, syloid or amorphous silica gel.
U.S. Pat. No. 4,956,241 to Chu et al discloses a slip-coated biaxially oriented film having good antiblocking properties. The slip coating comprises of (a) an aqueous wax emulsion or dispersion, (b) an aqueous polymer solution or emulsion with Tg between 30xc2x0-100xc2x0 C., and (c) a minute amount of talc or syloid.
U.S. Pat. No. 3,945,404 to Yamamatsu et al discloses a food casing having the inner surface thereof coated with a water-soluble chromium complex to enhance the release of processed meat from the casing.
U.S. Pat. No. 5,547,738 to Mitchell et al discloses liner less labels where the substrate has a pressure sensitive adhesive on one face and a release coating on the other. The preferred release coatings are formulations, which include silicone resins and chrome complexes of fatty acids.
U.S. Pat. No. 5,492,599 discloses a treated cellulose-based substrate e.g. paper with good release properties. The treated substrate is coated with a primer coating comprising a cationic polymer and with a release coating comprising a carboxy- or carboxylate-containing release polymer.
U.S. Pat. No. 2,273,040 describes Quilon(copyright), Werner-type chrome complexes useful for making a variety of substrates hydrophobic, oleophilic, and softer.
U.S. Pat. No. 3,484,271 to Kaliski et al describes a two-step process where a polyfunctional anionic component is applied followed by treatment with a polyfunctional cationic component (Quilon(copyright) Chrome Complex) to yield a surface adhesive to cooked food and plastic masses.
Japanese Examined Patent Application 63,075,199 (Kanzaki Paper) describes a water-soluble copolymer release agent for paper, with Tg of 60-20xc2x0 C., consisting of (a) 5 to 50% of a hydrophilic ethylenically unsaturated monomer, e.g., (meth)acrylic acid or maleic acid, (b) 20 to 95% of a (meth)acrylate monomer having 4-10 carbons, e.g. butyl or hexyl, and (c) 0 to 40% of another copolymerizable monomer, e.g. vinyl acetate, styrene or acrylonitrile. The release paper has excellent threading and release properties.
U.S. Pat. No. 4,226,749 describes a sizing composition with a cationic and anionic component mixture in a clay coating formulation.
U.S. Pat. No. 3,976,490 describes topical coating comprising a particulate material e.g. silica, CaCO3 in a polymeric binder adhered to the opaque plastomeric sheet material. The size of the particles and the thickness of the binder are selected to provide for the protuberance of at least a portion of the particles to act as spacers and thus function as the primary antiblocking component.
U.S. Pat. No. 5,959,031 issued to Thurgood Sep. 28, 1999 describes a polyamide film forming resin and at least one release agent material selected from the group consisting of N, Nxe2x80x2 ethylene bis amides of the formula R1xe2x80x94COxe2x80x94NHxe2x80x94CH2xe2x80x94CH2xe2x80x94NHxe2x80x94COxe2x80x94R2 wherein R1 is an aliphatic hydrocarbon chain of about 14 to about 42 carbon atoms, and R2 is a hydrogen atom or an aliphatic hydrocarbon chain of about 14 to about 42 carbon atoms, wherein the release agent material is present in an amount such that after the paste composite is formed, substantially all of the film can be removed from the surface of the composite.
U.S. Pat. No. 3,837,375 to Higgins et al describes a container used for packaging viscous tacky polymers by the process of hot filling. The latter containers have an inner lining of heat stabilized nylon coated with a silicone release agent; an uncoated cellophane film; a mineral pigment coated kraft paper overcoated with a silicone release agent; or kraft paper coated with finely divided mica. These containers are able to withstand hot packaging temperatures up to 450xc2x0 F. and at the same time permit the contents to be readily removed.
European Patent EP 0295375A2 discloses a silicone coated release film used in film impregnation of cyanate resin based prepegs in a continuous process. The release film is stripped from the advancing impregnated film while simultaneously one or more new release films are applied to the prepeg before, during or after impregnation. Apparently, the silicone coated release papers showed better release than those coated with QUILON(copyright) in the temperature range of 125xc2x0 C.-300xc2x0 C.
U.S. Pat. No. 5,858,487 to Boehler et al discloses a six layer microwaveable food wrap where the top layer is a non-stick coating for use in preventing food from adhering to a polymeric layer. The non-stick coating is made from a chrome complex of stearic acid ((chromium, pentahydroxy, (tetradecanoata)di-)), and is commercially available from E.I. du Pont de Nemours and Company as QUILON(copyright) C complex (both methyl cellulose and hydroxypropyl methylcellulose are recognized as acceptable food additives by the US Drug Administration (FDA) and are listed in the food chemicals codex alimentarius (Dow""s product literature)).
U.S. Pat. No. 4,735,860 discloses a heat transfer sheet, which prevents sticking and blocking problems and makes it possible to carry out printing smoothly. The latter sheet has hot-melt ink layer or one side and heat-resistant protective layer on the other. The heat-resistant protective layer comprises (a) thermoplastic resin having a COOH or OH group, (b) a polyamine or polyisocyanate, and a (c) a thermoplastic resin, or a composition based on a silicone-modified resin.
The various types of release materials can be categorized as waxes, such as petroleum waxes, vegetable waxes, animal waxes, and synthetic waxes; fatty acid metal soaps, such as metal stearates and others, for example, calcium ricinoleate; other long chain alkyl derivatives, fatty esters, fatty amides and amines, fatty acids and alcohols; polymers, such as polyolefins, silicones, fluoropolymers, natural polymers; others like poly(vinyl alcohol) and polyoxyalkylenes; fluorinated compounds and fluorinated fatty acids; and inorganic materials, such as silicates, talc, clays, kaolin, mica, and other particulates such as silica, graphite and carbon.
While all of the above references propose release coatings of various types, there remains a need for effective, inexpensive, high temperature, high humidity release coatings which can be applied to thermally resistant polymer films and which do not permanently transfer off the film to the surface in contact therewith.
Traditional release agents such as erucamide and polytetrafluoroethylene, which bloom to the surface in polyolefins, fail to do so in case of nylon films, such as polyamide66. Apparently, polyamide66 films have higher surface tension (43-50dyn/cm), can absorb up to 2% by weight of water and can be heated up to 150xc2x0 C. with no degradation. All these properties make polyamide66 film a friendly substrate for coating with water based coatings.
The disclosures of all documents, patents and applications referred to herein are incorporated herein by reference.
The present invention provides in a first aspect a coating composition for use as a surface coating for polymer release films for use in high temperature and/or high humidity applications, which comprises a solution of at least one hydroxypropyl methylcellulose having hydroxypropyl molar substitution of from 0 to about 0.82.
In another aspect, the invention provides a release coating composition as defined above, wherein the solution comprises from about 0.2% to about 40% by weight, preferably from about 0.2% by weight to about 15% by weight of low viscosity hydroxypropyl methylcellulose having hydroxypropyl molar substitution of from 0 to about 0.82 in water, wherein low viscosity means the viscosity of a 2% by weight of a solution of hydroxypropyl methylcellulose in water is up to 100 centipoise at room temperature (20xc2x0 C.). The hydroxypropyl methylcellulose is infinitely soluble in water and the maximum amount is determined by the coating equipment and cost limitations.
In another aspect, the invention provides a release coating composition as defined above, wherein the solution comprises up to about 3% by weight of high viscosity hydroxypropyl methylcellulose having hydroxypropyl molar substitution of from 0 to about 0.82 in water, wherein high viscosity means the viscosity of a 2% by weight of a solution of hydroxypropyl methylcellulose in water is from 100 to 100,000 centipoise at room temperature (20xc2x0 C.).
In another aspect, the invention provides a process for coating the surface of a polymer film to provide a release film for use in high temperature and/or high humidity conditions, which comprises coating at least one surface of the polymer film with a solution of a hydroxy propyl methyl cellulose having hydroxypropyl molar substitution of from 0 to about 0.82 to provide a coating weight of at least about 0.004 lb/ream per side and drying the coated film to set the coating. In another embodiment of this process, the film is coated on both sides in separate passes or in a single pass to achieve the desired coating weight.
In yet another aspect, the invention provides a release polymer film coated on at least one surface with hydroxypropyl methyl cellulose having hydroxypropyl molar substitution of from 0 to about 0.82. The release film may also be coated with a mixture of the hydroxypropyl methylcellulose having hydroxypropyl molar substitution of from 0 to about 0.82 and particulate solids.
The release coating of the present invention has been found to be useful in cured rubber manufacturing applications and also has utility in the manufacture of SMC, BMC and TMC, as well as fiberglass composites. In addition, it is useful in applications such as those described in U.S. Pat. No. 3,837,375 (packaging of hot, highly viscous, tacky polymers such as low molecular weight polystyrene); U S. Pat. No. 5,858,487 (laminated, non-stick food wraps); and U.S. Pat. No. 4,735,860 (therma-sensitive transfer sheets); as well as EP 0 295 375 (cyanate resin-based prepregs and films for use in advanced structural materials).
In another aspect the invention provides a process for curing rubber which comprises forming a sheet rubber layer in a calendar, laying layers of a release film as described above between layers of the sheet rubber, tightly overwrapping the stack of layers with a release film or cloth, before subjecting the stack of layers to elevated temperature in a dry or steam oven wherein the sheet rubber or sheet molding compound is cured and subsequently unwrapping the stacked, cured sheets.
In another aspect the invention provides a process for producing sheet molding composites which comprises:
(a) casting a layer of heat-curable thermosetting resin, in fluid form, onto a continuously advancing polymeric release film;
(b) introducing reinforcing material onto the advancing fluid layer;
(c) laying a polymeric film on the top surface of said reinforced fluid layer thereby forming a sandwich composite;
(d) advancing said sandwich composite through a series of kneading and compaction rolls; and
(e) winding the sandwiched composite into a roll for partial curing;
the improvement comprising using a release film as defined above. In a variant of this process a particulate solid is also present in the release coating composition.
In another aspect the invention provides a process for making thick molding composites, comprising
(a) introducing reinforcing material into a heat-curable thermosetting resin, in fluid form and mixing same until the material is mixed and wetted;
(b) casting a layer of said mixture onto a continuously advancing polymeric film;
(c) laying a polymeric film on the top surface of said reinforcing material-resin layer to form a sandwich composite;
(d) advancing the sandwich composite through at least one compaction roll;
(e) cutting the continuous lengths of the sandwich composite into lengths for partial curing;
the improvement comprising using a release film as defined above. Again, the release composition may also include a particulate solid.
In the following Table 1 there is set out the hydroxypropyl molar substitution of various grades of hydroxypropyl methylcellulose available commercially from the Dow Chemical Company, which have been found to be useful in the present invention.
The degree of hydroxypropyl substitution affects the viscosity of the methyl cellulose. Hydroxypropyl methyl cellulose grades can be classified into high viscosity and low viscosity grades. Low viscosity grades of hydroxypropyl methyl cellulose are those grades, which at solution concentration of about 2% in water at room temperature (20xc2x0 C.) result in the solution viscosity in the range of 0.1-100 centipoise. High viscosity grades of hydroxypropyl methyl cellulose are those grades, which at solution concentration of about 2% in water at room temperature (20xc2x0 C.) result in the solution viscosity in the range of 100-100,000 centipoise. The useful amounts for coating solutions of hydroxypropyl methylcellulose for use in the present invention range from about 0.2% to about 40% by weight, preferably from about 0.2% up to about 15% by weight. A most preferred range is from about 0.2 to about 6.0% by weight hydroxypropyl methylcellulose in water. The solutions of surface treated grades of hydroxypropyl methylcellulose available from Dow Chemical Company in water require pH adjustment in order to trigger the hydration process and subsequently, the viscosity build up. The latter trigger can be conveniently achieved by adding a small amount of base, such as ammonium hydroxide to the dispersion of surface treated hydroxypropyl methylcellulose in water.
The coating of the methylcellulose solution can be performed by rolling, dipping or spraying. The rolling method is preferred. Details of these coating methods are well known to those skilled in the art. Drying of the coated film is preferably by air drying, in a heated oven, at a temperature in the range of about 40xc2x0 C. to about 120xc2x0 C.
The humidity conditions under which the release film performs range from 0 to about 100% relative humidity. The high temperature conditions range from about room temperature (20xc2x0 C.) to about 210xc2x0 C.
The solution of methylcellulose may comprise a binary mixture of an organic solvent and water. Generally such a mixture preferably comprises about 2 to about 8 parts solvent per one part methylcellulose. An example of a preferred binary solution comprises from 0 to 35% by weight of alcohol, from about 0.2% to about 40% by weight of hydroxypropyl methylcellulose, and the remainder up to 100% by weight of water. There are a variety of organic solvents that may be used in such a binary mixture and the organic solvent may be selected from glycols, esters and amines. The Dow Technical Handbook for Methocel Cellulose Ethers referenced earlier contains a listing of suitable specific solvents. The solution may be prepared in concentrated form and then diluted to an appropriate concentration for the desired coating weight.
The methylcellulose solution may also contain particulate solids such as those mentioned above. Preferred are silica and talc. The ratio of particulate solid to methylcellulose is preferably in the range of from about 0.01 to about 1.5. The amount of organic solvent may range in this case up to about 50% by weight. The silica may be commercially available colloidal silica, examples of which are sold under the trade-marks Ludox(copyright), Bindzil(copyright) and Nyacol(copyright).
While the particulate solids act as processing aids, it has also been recognized that they facilitate higher transfer of the coating solution to the film. As a result when the particulate solid is present in the dried coating and in the preferred amount of from about 0.01 to about 0.60 by weight fraction, the amount of release agent required, namely the methylcellulose is reduced.
As is apparent from the subsequent Examples 12 to 16, a solution containing hydroxypropyl methyl cellulose and silica, for example, Methocel(copyright) E15LV/Ludox(copyright) CL-P ratio equal to 3/1, where Methocel(copyright) E15LV accounts for 1.86% to 3% of the solution, gives surprisingly higher transfer of the coating to a polymer film, such as Dartek(copyright) T404 above 2% total solid content of the solution. The latter effect viz. higher transfer of coating to the polymer film at 2% and the above total solids content in the solution can be seen in FIG. 4. Ludox(copyright) CL-P is an aqueous colloidal dispersion of 40% by weight of very small silica particles having 22 nm average particle diameter. The silica used in Ludox(copyright) CL-P is made up of negative silica particles with a positive layer of alumina. It is speculated that the positive charge imparted to the coatings of the invention by the Ludox(copyright) CL-P helps the transfer of the release coating onto the Dartek(copyright) T404 film, which has amide negative ions on the surface. For a particular coat weight of a two side coated Dartek(copyright) T404 film, 0.01-0.60 weight fraction of silica in the dried coating reduces the peel strength. The minimum in peel strength is found around 0.15 silica weight fraction. However, an excess amount of silica (above 0.60 weight fraction) on the surface increases the peel strength with complete adhesion (peel strength of 2000 g/1.27 cm from Viton(copyright) rubber compound) at 100% silica in the dried coating. The latter trend can be seen in FIG. 5.
The polymer film may be selected from polyolefins, polyesters, nylons and blends thereof. Nylon 66, Nylon 6 and polyester films are preferred. The films may be monoaxially or biaxially oriented. Generally any film having a softening point above the temperature of the application for the coating may be used. A preferred film is monoaxially oriented (in the machine direction) nylon, in particular nylon 66. A commercial example is Dartek T404 available from Enhance Packaging Technologies Inc. This film has good MD shrink properties at rubber curing conditions.